1. Field of the Invention
The present invention is related to a method for color correction, and particularly to a method for color correction which can modify the original luminance distribution state of image data. In this method, the fitting luminance is obtained by a fitting function of an interval and formed into a lookup table, and the modified gray signals can be quickly obtained from the lookup table to adjust the gray distribution state of display. The present invention is especially applicable to the color image processing of liquid crystal displays.
2. Description of the Prior Art
Liquid crystal display (LCD) is a planar display in opto-electrical products. It possesses many advantages such as small, light, thin, low power consumption, no flicker and no radiation etc. There are some problems such as overlong response time and afterimage due to properties of liquid crystal molecule such as viscosity, dielectric constant and elastic constant in image processing of display. Therefore, it is needed to improve image quality by improved driving methods wherein the active matrix driving method possesses the most industrial value among the driving methods. It has an additional active element at the intersection of the existing electrodes for disposing picture element in the active matrix driving method. One of the active matrix driving methods is the most popular thin film transistor (TFT) technology. There is an opposite electrode added to the intersection of the existing electrodes for disposing picture element and a thin film active element is placed at the intersection of the three electrodes on the panel of the TFT liquid crystal display.
Each picture element (pixel) of LCD panel can produce red, green and blue color (RGB). In fact, the liquid crystal has no color itself. Color is produced by light passing through the color filter on the LCD panel. The brightness of color is controlled by the quantity of the light passing through the pixel. The brightness scale unconcerned with the color filter is generally called “gray level”. The gray level is determined by the voltage produced by the driving element in the liquid crystal display. An 8 bits driving element can transmit 256 gray level voltages and the middle gray levels are determined by the middle voltages produced by the driving element. However, the light transmission of the liquid crystal is nonlinear, so the voltage must be modified to produce uniform distribution of gray level. The input and output relation of the voltage and pixel brightness is approximately a curve function: light output=(input voltage)γ, wherein γ (gamma) is a parameter. The corresponding relation of gray level and brightness (or luminance) is a curve called “gamma curve” as shown in FIG. 1A. The gamma curves of the three colors light of RGB corresponding to gray level are shown in FIG. 1B. The original gray level data of the gamma curve are generally normalized to produce gamma curves of three color lights with the same start point and end point, as shown in FIG. 1C. In general, the color correction of images is based on the normalized gamma curves.
In order to produce the best color display on the panel, i.e. the best gamma curve expression for a driving chip with a driving method, the luminance of the panel must first be measured by the color measurement system or device and the original gray level data are processed by the driving chip. In the prior technology, the color measurement system must measure 768 numbers of luminance for an 8 bits display panel to produce gamma curves. It will take a long time. Therefore, the present inventor invents a new and effective color correction method which can greatly shorten the color measurement time, speed up color production and make rich color.
Additionally, an 8 bits controller can produce 256 numbers of gray levels for liquid crystal display. Because a pixel has 256 numbers of gray levels for each primary color, so each pixel can display 256×256×256, i.e. 16,777,216 colors. However, it still can not satisfy human's demand for color. Although human's eye can not find the difference of color change in dynamic display on display, but in static display human's eye can find that the gray level change is not smooth enough. In order to solve this problem, a frame rate control (FRC) technology is then developed. In this technology, a picture is divided into several frames in a picture displaying time. When the luminance switches between the darker and the brighter in the switching process of image data, the pixel can produce middle luminance to make gray level expression smoother and make color display richer in the switching process. If two neighboring pixels open or close at the same time in the image color switching process, there will occur a flicker phenomenon which results in people's uncomfortableness. The two neighboring pixels must have two different gray levels for eliminating the flicker phenomenon. The method for producing different gray level is that the original gray level of the original gamma curve can be correlated to a target curve which could be established according to the characteristic of the LCD panel or one of the RGB gamma curve. The correlated gray level of target curve is then processed by FRC technology and then the corrected luminance is displayed on display device. In the prior art, the color processing method consists of the steps shown in FIG. 2. The luminance of the target curve corresponding to the original gray level of a color gamma curve is found by step 1 and step 2. Thereafter, the correlated luminance can be showed by gray level 128.5 of the original gamma curve according to step 3. Because the gray level 128.5 is between 128 and 129, it needs to extend 8 bits of operation to 9 bits to display the correspondent luminance by linear interpolation. If the middle gray level is between 128.5 and 129 or 128 and 128.5, the operation bit must be above 10 bits. It is a heavy burden to hardware, and the operation by linear interpolation can not be quick and correct to correspond to the target curve.
Therefore, the present inventor provides an improved method which can overcome the shortcomings stated above and make the image data processes more quickly and color displayed more rich.